The present invention relates to a cooling system for a fuel cell that generates electricity and also to a process for the operation of such a cooling system as well as to an electricity-generating group and an automobile vehicle comprising a fuel cell equipped with this cooling system.
Fuel cells are developed today in particular as a replacement for internal combustion engines in vehicles. Fuel cells obtain a better yield of energy than that of internal combustion engines by producing electricity used by an electrical drive motor.
Fuel cells generally comprise a stack of elementary cells comprising two electrodes separated by an electrolyte, and two conductive plates that supply the fuel and the oxidizer to the electrodes by internal conduits. The electrochemical reactions that take place upon contact with the electrodes generate an electric current and produce water while releasing heat energy that heats the different components.
In order to function correctly the fuel cells must be at a certain temperature range, depending on the type, between 60 and 800° C. The heat released by the starting of the reactions when the cell is cold serves, at first, to heat the cells in order bring them to the desired operating temperature.
In order to regulate the temperature of the cells, the fuel cells comprise a cooling system comprising a heat-conveying fluid circuit put in circulation by a pump that comes in contact with the fuel cells in order to absorb heat while warming up. The fluid then circulates in a heat exchanger in order to cool down, in particular by exchange with the ambient air.
A problem that is posed when starting a fuel cell that is at a temperature lower than 0° C. is that the water produced by the electrochemical reaction is at risk of freezing as long as this temperature is below this threshold of 0° C.
The fuel cell can not function correctly below this threshold and risks being destroyed. In order to remedy this problem a known cooling system presented in particular in EP0074701 comprises a cooling circuit comprising a first circulation loop with the heat exchanger and a pump that always delivers in the same direction, and a second circulation loop that traverses the cells.
The two circulation loops in EP0074701 intersect at a single point at a four-way valve that can be placed in two positions. Two of the four ways always serve as the entrance and the exit for the first circulation loop, and the two other ways allow the second loop to be arranged in series with the first loop in order to receive a circulation in one direction for one position of the valve and in the other direction for the other position of the valve.
While the pump is delivering in the first loop in a continuous manner in one and the same direction, an automatic actuation of the four-way valve from one position to the other allows the direction of the passage of the heat-conveying fluid to be alternated in the second loop and therefore in the cells.
Thus, a cold frequent alternating of the direction of the circulation of the fluid in the cells is realized with a circulation of the same reduced volume of fluid traversing these cells in one direction then in the other direction. The same volume of fluid exits on one side of the cells as a function of the discharge of fluid and of the frequency of alternation in order to reenter thereagain after the changing of the direction of circulation. The changes of the direction of circulation of the fluid are produced with intervals of time between two directions of circulation that are rather short in order to not allow each part of the fuel cell to reach its point of thermal stability.
The implementation of a low volume of fluid comprising an alternating movement that exchanges and distributes heat allows for better homogenization of the temperature at all points of the cells and between the cells situated at the center of the stack and those at the ends relative to a homogenization obtained with a movement of fluid in a single direction. Thus, a concentration of the heat that remains in the cells and in the parts of the conduits close to these cells is obtained, as the fluid does not circulate beyond these close parts.
Thus, a more rapid starting and rise of temperature of the fuel cell can be obtained before having to dissipate heat to the outside by the continuous manner of operation comprising a single direction of passage in which the fluid traverses the cells and passes into the heat exchanger in order to cool down.
A problem that is posed with this cooling circuit is that it is then necessary to size the pump with a greater power in order to put the heat-conveying fluid in motion in an alternating manner in the second loop. It will be noted that the energy necessary for putting in motion this heat-conveying fluid becomes all the more significant if this fluid comprises an antifreeze such as ethylene glycol or fluorinated oil, which is more viscous.
The pump then has a more significant bulkiness, mass and cost.
Another problem posed by this cooling circuit is that the four-way valve is relatively complex and expensive to manufacture. Moreover, the two loops intersecting at a single point form a specific circuit that is not always easy to realize in a simple manner starting from a conventional circuit comprising a single main loop.
Moreover, the manner of regulating the sequence of the changing of the direction of the fluid is based on global characteristics of the fuel cell whereas in the starting at a low temperature there can be significant temperature differences inside this fuel cell. Therefore, it is very difficult to make a good choice of the value of the frequency.